1. Field of the Invention
The present invention relates to a heat device suitable for use as a heat-fixing apparatus mounted to an electrophotographic image forming apparatus, and to an image forming apparatus including the heat device.
2. Description of the Related Art
In many cases, a heat roller type of heat device or a film heating type of heat device is used as a heat device (image heat-fixing apparatus) mounted to an image forming apparatus such as a copying machine and a laser beam printer.
The heat roller type of heat device includes a fixing roller (fixing member) having a halogen heater (heat source) mounted therein, and a pressure roller (pressure member) which is brought into contact with the fixing roller to form a nip portion. The heat device heats and fixes an unfixed toner image (developed image) formed on a recording material while nipping and conveying the recording material by the nip portion.
The film heating type of heat device is introduced in Japanese Patent Application Laid-Open No. S63-313182, Japanese Patent Application Laid-Open No. H02-157878, Japanese Patent Application Laid-Open No. H04-044075, Japanese Patent Application Laid-Open No. H04-204980, and the like. Specifically, in the heat device, a heat resistant fixation film (fixing member) is brought into close contact with a heater (heat source) made of ceramics by a pressure roller (pressure member) to slide and convey the fixation film, and the heater and the pressure roller form a press-contacting nip portion by nipping the fixation film. Then, an unfixed toner image (developed image) formed on a recording material is heated and fixed while the recording material is nipped and conveyed by the nip portion.
In the image forming apparatus including the heat roller type of heat device or the film heating type of heat device, it is known that, when printing is continuously performed on a recording material having a size smaller than a size of a heating area of the heater, an area of the heater through which the recording material does not pass (hereinafter, referred to as “non-sheet feeding area”) is excessively increased in temperature. When the temperature of the non-sheet feeding area of the heater is raised excessively, there is a fear that a thermal damage is given to components of the heat device.
Accordingly, in the image forming apparatus, when printing is continuously performed on the small-size recording material, in order to suppress the excessive rise of the temperature of the non-sheet feeding area of the heater, a control for making a print interval larger is performed to drastically decrease the output number of sheets (i.e., throughput), which is a processing ability per unit time.
Further, when printing is performed on a large-size recording material immediately after printing is continuously performed on the small-size recording material, it may cause a fixing failure such as hot offset occurs in the area in which the temperature of the non-sheet feeding area of the heater is raised. In order to prevent the phenomenon from occurring, it is necessary to take a given downtime until the temperature of the non-sheet feeding area becomes sufficiently low after the continuous printing is performed on the small-size recording materials.
Accordingly, as disclosed in Japanese Patent Application Laid-Open No. H05-134575 and Japanese Patent Application Laid-Open No. H10-177319, there is conventionally proposed a heat device in which an amount of heat generation of the non-sheet feeding area of the heater is reduced with respect to the area through which the recording material passes (sheet feeding area) when the continuous printing is performed on the small-size recording materials, both as the heat roller type of heat device and as the film heating type of heat device.
As described above, each of the heat devices disclosed in Japanese Patent Application Laid-Open No. H05-134575 and Japanese Patent Application Laid-Open No. H10-177319 includes two heaters, that is, a heater for a small-size recording material and another heater for a large-size recording material. The two heaters are separately controlled to be electrified, thereby enabling to change a turn-on duty ratio. As a result, it is possible to form heat generation distribution suitable for each size of recording materials.